Seconds in 1 month / (Total Hash Power MH/s / My Hash Power MH/s * Block Time s) So in my case with following parameters:

Seconds per month: 2592000

Total Hash power MH/s: 30728202.35

My hash power MH/s: 74.525

Block Time s: 15

I get 0.4190912262 as a result opposed to 2.10 value from online calculator mentioned above. I'm not entirely sure where my formula is wrong i.e. I calculate seconds needed to mine 1 block and then divide whole month by these seconds to find out how many blocks I can mine.